Documentation: fix description of ext4's block_validity mount option

Fix ext4 documentation according to commit 45f1a9c3
("ext4: enable block_validity by default")

Also fix some typos.

[ Further documentation cleanups by tytso ]

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>